The present invention relates to a radio conference system in which a hub station and a plurality of terminal stations interchange data with each other and, more particularly, to a terminal unit guiding technology for insuring a high transmission efficiency between the terminal stations and the hub station.
Some different transmission protocols have been proposed for a radio conference system in order to implement sure communication and high transmission efficiency between a single hub station and a plurality of terminal stations. For example, a polling system allows a hub station to send data to terminal stations and then confirm receipt terminal station by terminal station. Another system allows the individual terminal station to detect an error out of data received from a hub station and return a resend request signal (NAK) to the hub station. Further, a receipt terminal may include a display for displaying communication quality between it and a hub station in terms of a receipt electric field strength.
However, the conventional schemes described above each has some problems left unsolved, as follows. Although sure transmission is expected with the polling system, polling wastes time when it comes to a conference system in which radio communication is used at a visible distance. Specifically, this kind of conference system does not need strict confirmation of transmission and receipt because most terminal stations are given acceptable communication quality. Therefore, causing all the terminal stations to acknowledge receipt by polling needs an extra period of time and lowers the data transfer efficiency.
The NAK scheme saves the above station-by-station polling time by determining that a terminal station having returned no NAK signals within a preselected period of time has received data correctly. However, the problem with the NAK scheme is that when any one of many terminal stations is put in a low wave propagation condition, the hub station repeats resending exclusively for such a terminal station. Therefore, the hub station cannot send the next data to the terminal stations until it ends the resending operation meant for the terminal station in question, i.e., until it sends the data to all the terminal stations. As a result, if at least one of the terminal stations is of low propagation quality, the overall transmission efficiency of the system falls.
The display scheme capable of displaying the receipt electric field allows each terminal unit to be moved to a location where the wave propagation quality is relatively high in accordance with the display. Such mobility of the terminal unit increases the overall data transfer efficiency of the entire system. However, it is not easy even for an expert in the radio communications art to determine the degree of the receipt electric field strength which insures error-free data transmission. Moreover, in the case of packet transmission, the measurement of the electric field strength itself is sometimes difficult because the hub station does not always send an electromagnetic wave continuously.